Conventionally, the ink jet printer is known as an image forming apparatus which discharges the ink drop from the discharge nozzle of the head unit and forms an image on the recording sheet from the paper cassette.
In the ink jet printer, the ink drop discharged from the discharge nozzle directly reaches the sheet and the image is printed on the sheet. For this reason, in order to realize high quality of the image, it is necessary to raise the accuracy of positioning the ink drop on the recording sheet.
There have been some conceivable methods for raising the ink drop position accuracy: to keep the distance of the head and the sheet constant, to perform conveyance of the sheet with high accuracy, etc.
Japanese Laid-Open Patent Application No. 04-201469 and Japanese Laid-Open Patent Application No. 09-254460 disclose the method for conveying the sheet with high accuracy wherein the conveyance belt which conveys the sheet to the position where it confronts the head unit is charged uniformly, and the composition to electrostatically attach the sheet to the conveyance belt is provided.
However, as a result of charging the conveyance belt uniformly, when the sheet is electrostatically attached to the conveyance belt, the sheet is subjected to the dielectric polarization under the influence of the electric field of the conveyance belt. This dielectric polarization causes the charge with the polarity opposite to that of the conveyance belt to be generated at the conveyance belt side of the sheet, and causes the charge with the same polarity as that of the conveyance belt to be generated at the printing side of the sheet.
Simultaneously with this, the actual charge with the polarity opposite to the conveyance belt moves gradually from the interior of the sheet to the conveyance belt side of the sheet, and the actual charge with the same polarity as the conveyance belt moves gradually from the interior of the sheet to the printing side of the sheet. For this reason, while the charge on the conveyance belt and the charge on the side of the conveyance belt of the sheet are balanced gradually, the electric field of the conveyance belt become weaker and the amount of charge by the dielectric polarization generated on the sheet also decreases. By the time that the sheet is conveyed by the conveyance belt to the position where it confronts the head unit, most of the charge on the printing side of the sheet becomes the actual charge.
As shown in FIG. 21(a), the potential difference arises between the sheet on the conveyance belt 120 and the head 130 under the influence of the actual charge on the printing side of the sheet, and the electric field occurs. For this reason, under the influence of the electric field, the ink drop discharged from the discharge nozzle 131 of the head 130 is charged as shown in FIG. 21(b).
Consequently, the flight of the ink drop is disturbed under the influence of the electric field between the sheet and the head 130 and the impact position is thus deviated.
Moreover, as shown in (c) and (d) of FIG. 21, the ink mist flows backwards to the head 130, and the ink mist adheres to the discharge nozzle of the head 130, which will cause the problem in which the adhering ink interferes with the normal discharging of the ink from the head 130.
To obviate the problem, Japanese Laid-Open Patent Application No. 2003-103857 discloses the composition wherein the AC bias voltage is applied to the conveyance belt and the conveyance belt is charged alternately to one of the positive polarity and the negative polarity.
However, in the case of charging the conveyance belt alternately to one of the positive polarity and the negative polarity, a non-uniform electric field is generated in the perpendicular direction from the positive charge on the conveyance belt to the conveyance belt, but the electric field in the middle thereof is deviated toward the negative electric charge on the conveyance belt.
Since the electric field in the closed formation is generated on the conveyance belt, the influence of the electric field from the conveyance belt on the printing side of the sheet becomes weaker. Consequently, the amount of charge generated on the printing side of the sheet decreases.
Furthermore, if an extended time passes, the positive charge and the negative charge which move to the printing side of the sheet are attracted and canceled by each other. Consequently, by the time that the sheet is conveyed to the position where it confronts the head unit, any of the charge almost does not exist on the printing side of the sheet.
For this reason, the potential difference between the sheet and the head unit no longer arises, and the electric field does not occur. Therefore, it is suppressed that the ink drop is charged and the flight of the ink drop is disturbed and the impact position is deviated, or the ink mist flows backwards and adheres to the discharge nozzle of the head.
In order to eliminate the actual charge on the printing side of the sheet, it is necessary that a certain time passes. For this reason, in order to eliminate the actual charge on the sheet to an extent that the potential difference does not occur even if the sheet is conveyed to the position where it confronts the head unit, it is necessary to secure the time from the instant the sheet is electrostatically attached to the conveyance belt to the instant the sheet arrives at the position where it confronts the head unit.
Conventionally, if the conveyance speed of the sheet is raised for improvement of the printing speed, the actual charge on the printing side of the sheet cannot be eliminated by the time the sheet arrives at the position where it confronts the head unit. Therefore, the charge remains on the printing side of the sheet and the electric field occurs between the sheet and the head unit. For this reason, there is the problem that the impact position of the ink drop is deviated, or the ink mist adheres to the head discharge nozzle, and it is difficult to obtain the high quality image with the conventional image forming apparatus.